The invention relates to a device for measuring relative position error between a laser beam and a target, and also to applications of the device to making a machine for exposing printed circuit panels.
In order to manufacture printed circuits by conventional methods, a known step consists in covering the conductive layer of the printed circuit panel in a layer of resist material and in exposing said resist layer through an artwork so as to enable the shape of the desired conductor tracks to be obtained on the printed circuit. After the layer of resist has been exposed, non-exposed zones are removed, which zones correspond to those portions of the conductive layer that are to be removed.
Another known technique which is becoming more widespread consists in making printed circuit panels in which the resist layer is imprinted locally by impact from a laser beam. The position of the laser beam impact on the panel is controlled as a general rule by a scanning assembly comprising a rotary polygonal mirror combined with a control laser beam interrupter device which is usually constituted by an acousto-optical modulator.
Other techniques that also use a laser beam consist either in ablating the layer of resist directly by means of a laser beam, or else in not using a layer of resist at all, and in ablating the conductive layer directly by means of a laser beam.
Since the number of laser beam impact points is naturally very large in order to achieve the precision required on a printed circuit panel of large size, it is advantageous to be able to subdivide the surface of the printed circuit panel into a plurality of zones and to scan each of the zones simultaneously by means of a respective laser beam.
In machines which enable that operation to be performed and which are referred to below as xe2x80x9cexposure machinesxe2x80x9d, two main relative positioning problems arise. Firstly, it is necessary to calibrate the position of the laser beam or of each laser beam accurately relative to the structure of the exposure machine, and secondly it is necessary to position the printed circuit panel that is to be exposed by the laser beam with very great accuracy on the structure of the machine.
For laser beam calibration, it will be understood that, in the machine, the beam is directed towards the printed circuit panel through an optical unit which includes a large number of optical components. In addition, the printed circuit panel is scanned as a general rule with the help of a rotary polygonal mirror in which the rotation of each face defines a portion of a scan length. Furthermore, the pattern that is to be made using the laser beam is defined by means of stored information defining for each point on the panel either that it should be impacted or else that it should not be impacted.
In the light of all the above, it will be understood that however accurately the various components are adjusted initially, it is necessary prior to each use of the machine to verify the real position of the impact of a laser beam relative to the structure of the machine.
For positioning the panel relative to the structure of the machine, it will be understood that this also must be performed with very great accuracy, particularly when the printed circuit panels are designed to form component elements of a multilayer printed circuit. It is recalled that printed circuit panels are usually positioned relative to the structure of the machine by means of circular holes formed at certain locations in the panel that are defined with precision.
An object of the present invention is to provide a device for measuring relative position error between a laser beam and a target which can be constituted, for example, by the structure of an exposure machine or by a printed circuit panel, which device makes it simple to determine error with very great accuracy.
According to the invention, to achieve this object, in a device for measuring relative position error between a laser beam and a target:
said target is provided with a circular orifice occupying a reference position relative to said target; and
the device further comprises:
means for focusing the laser beam in the plane of said target; and
means for taking an image of said circular hole and of the beam focused in the plane of said target, and means for calculating the components of the segment between said point of impact and the center of said hole.
It will be understood that by means of the invention the error is indeed determined very precisely, in particular because the image-taking means, e.g. a camera, do not need to be positioned with precision since it suffices that they enable an image to be obtained simultaneously of the periphery of the circular hole and of the impact of the laser beam.
Another object of the invention is to provide an application of the above-defined device to making a machine for exposing printed circuit panels in order to position a printed circuit panel relative to the structure of the machine.
In this application said machine further comprises:
means for focusing said laser beam successively on two corresponding points of the structure which ought to be occupied by the centers of the two circular holes;
camera means for taking respective images of the two holes together with the corresponding points of impact of the laser beams;
calculator means responsive to each of said images to calculate the components of the segment joining the center of each circle to the point of impact of the corresponding beam, thereby obtaining two data sets concerning the position error of the panel; and
panel-displacement means for displacing said panel relative to the structure, at least in rotation, and control means for controlling the panel-displacement means and the means for focusing the laser beam as a function of the two data sets concerning the position error of the panel.
It will be understood that under such circumstances, the reference is constituted by the point of impact of the laser beam and that the element whose positioning error is to be corrected is the panel together with its two positioning holes.
Another object of the invention is to apply the above-defined detection device to calibrating the laser beam of a machine for exposing printed circuit panels. This application further comprises:
at least one target secured to said structure, said target including at least one circular hole whose center constitutes a reference point of the structure;
means for controlling the beam deflection means on the basis of position information corresponding to the position of the center of said circle;
camera means for taking an image of said circular hole and of the actual point of impact of the laser beam in said hole;
calculator means for calculating the components of the segment joining the center of said hole to the point of impact of the laser beam; and
means for transmitting said components to the control means for controlling the beam-deflector means.
It will be understood that under such circumstances, the position reference is constituted by the target provided with the circular hole and the element whose position error is to be determined is the point of impact of the laser beam.